1. Field of the Invention
The present invention relates to the field of dielectric resonator filters in mobile and satellite commutations. The present invention specifically relates to filters utilizing a combination of TE and modified HE mode dielectric resonators.
2. Description of the Prior Art
Dielectric resonator filters play an important role in mobile and satellite communications. Two types of dielectric resonator filters are commonly used. One type is a dual mode dielectric resonator filter, which operates in a HE 11 mode and provides low loss, smaller volume and elliptic function realizations. The inferior spurious characteristics in the output are the drawback of this type of dual mode dielectric resonator filter. The other type is a single mode dielectric resonator loaded filter with all resonators operating in a TE 01 mode, which provides low loss and good spurious free performance. In order to get the elliptic function characteristics in these types of filters, electric coupling between non-adjacent resonators is needed.
In dielectric resonator filters, the dominant coupling between dielectric resonators is magnetic in nature, which can be coupled between dielectric resonators (DR) using easily tunable irises. In the case of elliptic filters, there is a need to couple the electric field between non-adjacent dielectric resonators, which in the conventional approach needs some additional metallic member between the dielectric resonators. In the prior art, the electric field coupling between dielectric resonator pucks are achieved through a ground isolated coaxial probe method and a bar coupling method.
FIG. 1 shows a ground isolated probe method, which is the most commonly used method for realization of the electric field coupling between TE mode dielectric resonator pucks 2 and 4, which are separated by a metal separating wall 3. In this method, a metal probe 7 is placed in a suitable manner near the dielectric resonator pucks 2 and 4 to be coupled. The metal probe 7 is mounted between the metal cavities 1 and 5 by a conventional process as would be understood by one skilled in the art. This metal probe 7 is isolated from the metal cavities 1 and 5 by a suitable dielectric material 6, such as polytetrafluoroethylene (PTFE), e.g, TEFLON®. The probe dimensions become smaller with an increase in frequency and the probe fabrication, as well as the assembly becomes increasingly difficult. Thus, the metal probe 7 is an additional component. Moreover, the assembled probe coupling cannot be tuned and some other components and processes have to be employed for fine-tuning the required coupling. The metal probe 7 is normally placed very close to the dielectric resonator pucks 2 and 4, and hence limits the designs from spurious modes and high power point of view.
FIG. 2 shows a bar coupling method for realization of the electric field coupling between TE mode dielectric resonator pucks 22 and 24 in respective cavities 21 and 25. The coupling bar 26 made from a nickel-iron alloy material such as FeNi36 (64FeNi in the US) (e.g., INVAR®) is normally required for applications requiring operation over a large temperature range. This bar coupling can solve the tuning problem to some extent, but this is still an additional component required for the desired coupling. For low coupling values, the gaps between the wall 23 and the coupling bar 26 is low, which restricts suitable modification in the coupling arrangement for high power applications. Further, the filters utilize mixed mode electric couplings to use two HE mode resonators 35 and 36. Coupling between TE mode resonators 32 and 34 i.e. TE-TE coupling as well as TE-HE coupling is a positive coupling where as HE-HE coupling is a negative coupling. In a conventional mixed mode coupling, an Iris 37 is used between the HE mode resonators 35 and 36 for negative coupling in the same manner as is used for positive coupling between two TE mode resonators 32 and 34, as shown in FIG. 3. Hence, the negative coupling is also fully tunable in this method. However, there are many problems associated with this conventional implementation of the mixed mode coupling. In particular, the HE dielectric resonators 35 and 36 have two degenerate modes at the same frequency and are exploited well for dual mode dielectric resonator (DR) filters, but it is very difficult to separate in the single mode filters. Moreover, the size and weight of HE mode dielectric resonators 35 and 36 is high as compared to the TE mode dielectric resonators 32 and 34. Also shown in FIG. 3 are a metal wall 31 configured for defining metal cavities and separating walls 33 disposed between the resonators 32, 34, 35 and 36.
With respect to the conventional approaches, additional circuit components are utilized for realization of the electric field coupling between TE mode dielectric resonator pucks. However, in mixed mode electric couplings, these approaches result in an increase in the size and weight of the HE mode dielectric resonators. It is very difficult to separate the two degenerate modes of the resonators. Therefore, it is essential to provide an electrical field coupling between a pair of non-adjacent dielectric resonators operating in a single mode filter without using any additional component.